The present invention relates to an article comprising a substrate containing silicon and at least one barrier layer which functions as a thermal barrier and, more particularly, a layer which is effective at temperatures in excess of 1300xc2x0 C. in aqueous environments.
Ceramic materials containing silicon and metal alloys containing silicon have been proposed for structures used in high temperature applications as, for example, gas turbine engines, heat exchangers, internal combustion engines, and the like. A particular useful application for these materials is for use in gas turbine engines which operate at high temperatures in aqueous environments. It has been found that these silicon containing substrates can recede and lose mass as a result of a formation volatile Si species, particularly Si(OH)x and SiO when exposed to high temperature, aqueous environments. For example, silicon carbide when exposed to a lean fuel environment of approximately 1 ATM pressure of water vapor at 1200xc2x0 C. will exhibit weight loss and recession at a rate of approximately 6 mils per 1000 hrs.
Thermal layers for high temperature aqueous/steam environments have been developed which are useful at temperatures up to 1300xc2x0 C. Naturally it would be highly desirable to have effective thermal barrier layers for silicon containing substrates which operate in aqueous environment at temperatures in excess of 1300xc2x0 C.
Furthermore, it is not uncommon to use one or more layers on a substrate such as bond layers, intermediate barrier layers, and the like. It is important for the integrity of the final article to maintain a compatibility between the coefficient of thermal expansion (CTE) of the adjacent silicon containing substrate and/or the thermal barrier layer(s). If the CTE between adjacent layers and substrate are not compatible, the overall coating can break down at the interfaces between the non-compatible layers.
Accordingly, it is a principle object of the present invention to provide an article comprising the silicon containing substrate and at least one thermal barrier layer wherein the layer protects the silicon substrate when exposed to temperatures in excess of 1300xc2x0 C. in aqueous environments and has a coefficient of thermal expansion which is compatible with the substrate when adjacent to the substrate and compatible with any other adjacent layer or layers.
Further objects and advantages of the present invention will appear hereinbelow.
The present invention relates to an article comprising a silicon containing substrate having at least one thermal barrier layer wherein the layer functions to both inhibit the formation of undesirable gaseous species of silicon when the article is exposed to a high temperature, aqueous environment and to provide thermal protection. By high temperatures is meant temperatures in excess of 1300xc2x0 C. By aqueous environment is meant a water and/or steam environment. The silicon containing composite is preferably a ceramic or metal alloy containing silicon. The thermal barrier layer is characterized by a coefficient of thermal expansion which is within plus or minus 3.0 ppm per degree centigrade of the coefficient of thermal expansion (CTE) of the silicon containing substrate and/or any adjacent barrier layer.
The layers of the present invention contain TaOx (where x=1 to 3 and is a ratio of 0 to the compound element, see U.S. Pat. No. 4,155,016), for example, Ta2O5 and an oxide selected from the group consisting of MgO, CaO, SrO, BaO, Al2O3, Y2O3, La2O3, Re2O3 (where Re is a rare earth element) and mixtures thereof. It has been found in accordance with the present invention that Ta2O5 gives to the barrier layer the desired high temperature capability. The CTE compatibility is accomplished by providing a CTE tailoring additive in the layer in an amount sufficient to maintain the desired CTE compatibility with the substrate and adjacent layers. In this regard see co-pending Application Ser. No. 10/034,677, filed concurrently herewith. NbOx (where x=1 to 3 and is a ratio of 0 to the compound element, see U.S. Pat. No. 4,155,016) is, in accordance with the present invention, a suitable CTE tailoring additive.
In a preferred embodiment of the present invention the article can include one or more intermediate layers between the silicon based substrate and the thermal barrier layer. The intermediate layer(s) serve(s) to provide enhanced adherence between the barrier layer and the substrate and/or to prevent reactions between the barrier layer and the substrate. These layers contain, if necessary, the CTE tailoring additive so as to insure compatibility between the layers as noted above.